Method for hydrotreating heavy hydrocarbon feedstocks using permutable reactors, including at least one step of short-circuiting a catalyst bed

ABSTRACT

Process for hydrotreating a heavy hydrocarbon fraction using a system of switchable fixed bed guard zones each containing at least two catalyst beds and in which whenever the catalyst bed that is brought initially into contact with the feed is deactivated and/or clogged during the steps in which the feed passes successively through all the guard zones, the point of introduction of the feed is shifted downstream. The present invention also relates to an installation for implementing this process.

The present invention relates to a process for hydrotreating a heavyhydrocarbon fraction using a system of switchable fixed bed guard zoneseach containing at least two catalyst beds and in which whenever thecatalyst bed that is brought initially into contact with the feed isdeactivated and/or clogged during the steps in which the feed passessuccessively through all the guard zones, the point of introduction ofthe feed is shifted downstream. The present invention also relates to aninstallation for implementing this process.

Hydrotreating of hydrocarbon feeds is becoming increasingly important inrefining practice with the increasing need to reduce the quantity ofsulphur in petroleum cuts and to convert heavy fractions to lighterfractions, which can be upgraded as fuels and/or chemical products. Itis in fact necessary, in view of the standard specifications imposed byeach country for commercial fuels, for imported crudes, which havehigher and higher contents of heavy fractions, of heteroatoms and ofmetals, and lower and lower hydrogen contents, to be upgraded as far aspossible.

Catalytic hydrotreating makes it possible, by bringing a hydrocarbonfeed into contact with a catalyst in the presence of hydrogen, to reduceits content of asphaltenes, metals, sulphur and other impuritiesconsiderably, while improving the ratio of hydrogen to carbon (H/C) andwhile transforming it more or less partially into lighter cuts. Thus,hydrotreating (HDT) in particular means reactions ofhydrodesulphurization (HDS) by which are meant the reactions forremoving sulphur from the feed with production of H₂S, reactions ofhydrodenitrogenation (HDN) by which are meant the reactions for removingnitrogen from the feed with production of NH₃, and reactions ofhydrodemetallization by which are meant the reactions for removingmetals from the feed by precipitation, but also hydrogenation,hydrodeoxygenation, hydrodearomatization, hydroisomerization,hydrodealkylation and hydro-deasphalting.

There are two types of hydrotreating process for treating heavy feedssuch as atmospheric residues (AR) or vacuum residues (VR): fixed bedprocesses and ebullating bed processes. Zong et al. (Recent Patents onChemical Engineering, 2009, 2, 22-36) summarize the various processesknown in the treatment of heavy petroleum feeds.

The technology of the fixed bed processes has found the widestindustrial application owing to its technical maturity, lower cost andstable and reliable performance. In these processes, the feed circulatesthrough several fixed bed reactors arranged in series, the firstreactor(s) being used in particular for performing hydrodemetallizationof the feed (so-called HDM step) as well as a proportion ofhydrodesulphurization, the last reactor(s) being used for performingdeep refining of the feed (hydrotreating step, HDT), and in particularhydrodesulphurization (so-called HDS step). The effluents are withdrawnfrom the last HDT reactor.

The fixed bed processes lead to high performance in refining (productionof 370° C.⁺ cuts with less than 0.5 wt. % of sulphur and containing lessthan 20 ppm of metals) from feed containing up to 5 wt. % of sulphur andup to 300 ppm of metals, in particular nickel and vanadium). The variouseffluents thus obtained can serve as a basis for the production of heavyfuel oils of good quality, of gas oil and gasoline, or feeds for otherunits such as catalytic cracking.

Beyond this content of metals, it is known that the first catalyst bedscan quickly be deactivated because of the considerable deposit of metalsthat is produced. To compensate for this deactivation, the temperatureof the reactor is then increased. However, this increase in temperaturepromotes the deposition of coke, accelerating the processes ofintragranular clogging (plugging of the catalyst pores) andextragranular clogging (plugging of the catalyst bed). Beyond thesecontents of metals in the feed, ebullating bed processes are thusgenerally preferred. In fact, one problem posed by fixed bed catalytichydrotreating of these feeds arises because during the hydrotreatingreactions of petroleum fractions containing organometallic complexes,most of these complexes are destroyed in the presence of hydrogen,hydrogen sulphide, and a hydrotreating catalyst. The metal constituentof these complexes then precipitates in the form of a solid sulphide,which will adhere to the catalyst. This is particularly so withcomplexes of vanadium, nickel, iron, sodium, titanium, silicon andcopper, which are naturally present in crude oils at a varying leveldepending on the origin of the petroleum, and which, during theoperations of distillation, tend to become concentrated in high boilingpoint fractions and in particular in residues. In addition to theseimpurities, coke is also deposited, and together they then tend todeactivate and clog the catalytic system very quickly. These phenomenalead to stoppage of the hydrotreating units for replacing the solids andto an overconsumption of catalyst, which a person skilled in the artwishes to minimize.

Another problem posed by fixed bed catalytic hydrotreating of thesefeeds is clogging. It is known that catalyst beds, in particular theupper portions of catalyst beds, and more particularly the upperportions of the first catalyst bed in contact with the feed, are likelyto clog quite quickly because of the asphaltenes and sediments containedin the feed, which is manifested firstly by an increase in head loss andsooner or later requires a stoppage of the unit for replacing thecatalyst.

Therefore it becomes necessary to stop the unit in order to replace thefirst catalyst beds, which are deactivated and/or clogged. Thehydrotreating processes for feeds of this type must therefore bedesigned so as to permit an operating cycle that is as long as possiblewithout stopping the unit.

STATE OF THE ART

There have been attempts to resolve these drawbacks of the fixed bedarrangements in various ways, in particular by using guard bedsinstalled upstream of the main reactors. The main task of the guard bedsis to protect the catalysts of the main HDM and HDT reactors downstream,by performing a proportion of the demetallization and by filtering theparticles contained in the feed that can lead to clogging. The guardbeds are generally integrated in the HDM section in a process forhydrotreating heavy feeds generally including a first HDM section andthen a second HDT section. Although the guard beds are generally usedfor performing a first hydrodemetallization and a filtration, otherhydrotreating reactions (HDS, HDN, etc.) will inevitably take place inthese reactors owing to the presence of hydrogen and a catalyst.

Thus, installation of one or more moving-bed reactors at the head of theHDM step has been considered (U.S. Pat. No. 3,910,834 or GB2124252). Theoperation of these moving beds can be co-current (SHELL's HYCON processfor example) or counter-current (OCR process of Chevron Lummus Globaland the applicant's HYVAHL-M™ process for example).

Adding a fixed bed guard reactor in front of the HDM reactors has alsobeen considered (U.S. Pat. No. 4,118,310 and U.S. Pat. No. 3,968,026).Most often this guard reactor can be by-passed in particular by using anisolating valve. This provides temporary protection of the main reactorsagainst clogging.

Moreover, a system has also been described, in particular by theapplicant (FR2681871 and U.S. Pat. No. 5,417,846), for combining thehigh performance of the fixed bed with a high operating factor fortreating feeds with high contents of metals, which consists of ahydrotreating process in at least two steps for a heavy hydrocarbonfraction containing asphaltenes, sulphur-containing impurities andmetallic impurities, in which, during the first so-called HDM step, thefeed of hydrocarbons and hydrogen is passed, under conditions of HDM,over an HDM catalyst, then, during the next, second step, the effluentfrom the first step is passed, under conditions of HDT, over an HDTcatalyst. The HDM step comprises one or more fixed bed HDM zonespreceded by at least two guard HDM zones, also called “switchablereactors”, also of fixed bed design, arranged in series to be usedcyclically consisting of successive repetition of steps b) and c)defined below:

a) a step in which the guard zones are all used together for a period atmost equal to the deactivation time and/or clogging time of one of them,b) a step during which the deactivated and/or clogged guard zone isby-passed and the catalyst that it contains is regenerated and/orreplaced with fresh catalyst and during which the other guard zone(s)are used,c) a step during which the guard zones are all used together, the guardzone of which the catalyst was regenerated during the preceding stepbeing reconnected and said step being continued for a period at mostequal to the deactivation time and/or clogging time of one of the guardzones.

This process, known by the name HYVAHL-F™, can provide an overalldesulphurization greater than 90% and an overall demetallization of theorder of 95%. The use of switchable reactors permits continuous cyclicoperation.

It has now been discovered, surprisingly, that it is possible toincrease the time of use of the switchable reactors before replacementof the catalyst contained in a switchable reactor becomes necessary. Thepresent invention thus improves the performance of switchable reactorsas described by the applicant in patent FR2681871 by integrating intothis process at least two catalyst beds in each switchable reactor andby integrating into certain steps of the process at least one step ofby-passing deactivated and/or clogged catalyst beds, also called aby-pass step.

In the catalyst beds, clogging occurs a priori in the upper portions ofthe catalyst beds, and in particular in the upper portions of the firstcatalyst bed brought into contact with the feed in the direction offlow. The same applies to deactivation of the catalyst (deposition ofmetals). According to the invention, whenever a catalyst bed isdeactivated and/or clogged, this catalyst bed is by-passed and the pointof introduction of the feed is shifted relative to this bed downstreamonto the next catalyst bed, not yet deactivated and/or clogged, of thesame switchable reactor. Thus, by successive by-pass steps of the mostclogged and/or deactivated portion(s) of the reactor, the volume of eachswitchable reactor is fully utilized until it is exhausted (i.e. untilits last catalyst bed is also deactivated and/or clogged), whilemaintaining the cyclic operation of the switchable reactors. Thus, thebed(s) downstream of the deactivated and/or clogged bed(s) of the samereactor are used for a longer time. This has the effect of increasingthe duration of each step of the cycle of the switchable reactors duringwhich the feed passes successively through all the reactors, whichprovides a longer operating cycle of the switchable reactors.

This lengthening of the cycle leads to an increase in the operatingfactor of the unit as well as a saving of time, a reduction of operatingcosts and a reduction of the consumption of fresh catalyst. The aim ofthe present application is thus to increase the cycle time of theswitchable reactors.

DETAILED DESCRIPTION

The present invention provides an improvement of the hydrotreatingprocess carried out using guard zones (switchable reactors) as describedin patent FR2681871. The operation of the guard zones according toFR2681871 is described in FIG. 1, comprising two guard zones (orswitchable reactors) R1 a and R1 b. This hydrotreating process comprisesa series of cycles each comprising four successive steps:

-   -   a first step (called “step a” hereinafter) during which the feed        passes successively through reactor R1 a, then reactor R1 b,    -   a second step (called “step b” hereinafter) during which the        feed only passes through reactor R1 b, reactor R1 a being        by-passed for catalyst regeneration and/or replacement,    -   a third step (called “step c” hereinafter) during which the feed        passes successively through reactor R1 b, then reactor R1 a,    -   a fourth step (called “step d” hereinafter) during which the        feed only passes through reactor R1 a, reactor R1 b being        by-passed for catalyst regeneration and/or replacement.

During step a) of the process, the feed is introduced via line 3 andline 21, having an open valve V1, into line 21′ and the guard reactor R1a containing a fixed catalyst bed A. During this period, valves V3, V4and V5 are closed. The effluent from reactor R1 a is sent via pipe 23,pipe 26, having an open valve V2, and pipe 22′ into the guard reactor R1b containing a fixed catalyst bed B. The effluent from reactor R1 b issent via pipes 24 and 24′, having an open valve V6, and pipe 13 to themain hydrotreating section 14.

During step b) of the process, valves V1, V2, V4 and V5 are closed andthe feed is introduced via line 3 and line 22, having an open valve V3,into line 22′ and reactor R1 b. During this period the effluent fromreactor R1 b is sent via pipes 24 and 24′, having an open valve V6, andpipe 13 to the main hydrotreating section 14.

During step c), valves V1, V2 and V6 are closed and valves V3, V4 and V5are open. The feed is introduced via line 3 and lines 22 and 22′ intoreactor R1 b. The effluent from reactor R1 b is sent via pipe 24, pipe27, having an open valve V4, and pipe 21′ to the guard reactor R1 a. Theeffluent from reactor R1 a is sent via pipes 23 and 23′, having an openvalve V5, and pipe 13 to the main hydrotreating section 14.

During step d), valves V2, V3, V4 and V6 are closed and valves V1 and V5are open. The feed is introduced via line 3 and lines 21 and 21′ intoreactor R1 a. During this period the effluent from reactor R1 a is sentvia pipes 23 and 23′, having an open valve V5, and pipe 13 to the mainhydrotreating section 14.

The cycle then begins again. The operations on the valves of the unitenabling the functioning of the switchable reactors according toFR2681871 are presented in Table 1.

TABLE 1 Operations on the valves around the switchable reactorsaccording to FR2681871 (without external by-pass) Step in CycleIntervention V1 V2 V3 V4 V5 V6 a R1A + R1B — O* O C** C C O b R1B R1A CC O C C O c R1B + R1A — C C O O O C d R1A R1B O C C C O C a R1A + R1B —O O C C C O *O = open, **C = closed

According to the present invention, additional by-pass steps ofdeactivated and/or clogged catalyst beds (steps a′ and c′) in the stepsof the cycle during which the feed passes successively through the tworeactors (steps a) and c)), are added to the process steps as describedabove.

More particularly, the present invention relates to a process forhydrotreating a heavy hydrocarbon fraction containing asphaltenes,sediments, sulphur-containing, nitrogen-containing and metallicimpurities, in which the feed of hydrocarbons and hydrogen is passed,under conditions of hydrotreating, over a hydrotreating catalyst, in atleast two fixed bed hydrotreating guard zones each containing at leasttwo catalyst beds, the guard zones being arranged in series to be usedcyclically, consisting of successive repetition of steps b), c) and c′)defined below:

-   -   a step a) during which the feed passes through all the catalyst        beds of the guard zones for a period at most equal to the        deactivation time and/or clogging time of a guard zone,    -   a step a′) during which the feed is introduced, by-passing the        deactivated and/or clogged catalyst bed, onto the next catalyst        bed not yet deactivated and/or clogged of the same guard zone        for a period at most equal to the deactivation time and/or        clogging time of a guard zone, step a′) being repeated until the        feed is introduced onto the last catalyst bed not yet        deactivated and/or clogged of the same guard zone for a period        at most equal to the deactivation time and/or clogging time of a        guard zone,    -   a step b) during which the deactivated and/or clogged guard zone        is by-passed and the catalyst that it contains is regenerated        and/or replaced with fresh catalyst and during which the other        guard zone(s) are used,    -   a step c) during which the feed passes through all the catalyst        beds of the guard zones, the guard zone of which the catalyst        was regenerated during the preceding step being reconnected so        as to be downstream of all the other guard zones and said step        being continued for a period at most equal to the deactivation        time and/or clogging time of a guard zone,    -   a step c′) during which the feed is introduced onto the next        catalyst bed not yet deactivated and/or clogged of the same        guard zone for a period at most equal to the deactivation time        and/or clogging time of a guard zone, step c′) being repeated        until the feed is introduced onto the last catalyst bed not yet        deactivated and/or clogged of the same guard zone for a period        at most equal to the deactivation time and/or clogging time of a        guard zone,    -   a step d) during which the deactivated and/or clogged guard zone        is by-passed and the catalyst that it contains is regenerated        and/or replaced with fresh catalyst and during which the other        guard zone(s) are used.

The guard zones, in particular the first guard zone brought into contactwith the feed, gradually become laden with metals, coke, sediments andvarious other impurities. When the catalyst or catalysts that theycontain is/are practically saturated with metals and various impurities,the zones must be disconnected for carrying out replacement and/orregeneration of the catalyst(s). Preferably, the catalysts are replaced.This moment is called the deactivation time and/or clogging time.Although the deactivation time and/or clogging time varies in relationto the feed, the operating conditions and the catalyst(s) used, it isgenerally manifested by a drop in catalyst performance (an increase inthe concentration of metals and/or other impurities in the effluent), anincrease in the temperature required for maintaining constanthydrotreating or, in the specific case of clogging, by a significantincrease in head loss. The head loss Δp, expressing a degree ofclogging, is measured continuously throughout the cycle on each of thezones and can be defined by an increase in pressure resulting frompartially blocked passage of the flow through the zone. The temperatureis also measured continuously throughout the cycle on each of the twozones. In order to define a deactivation time and/or clogging time, aperson skilled in the art first defines a maximum tolerable value of thehead loss Δp and/or of the temperature as a function of the feed to betreated, the operating conditions and catalysts selected, and startingfrom which it is necessary to proceed to by-passing of a catalyst bed orto disconnection of the guard zone. The deactivation time and/orclogging time is thus defined as the time when the limit value of headloss and/or of temperature is reached. As a general rule the limit valueof head loss and/or of temperature is confirmed during initialcommissioning of the reactors. In the case of a process forhydrotreating heavy fractions, the limit value of head loss is generallybetween 0.3 and 1 MPa (3 and 10 bar), preferably between 0.5 and 0.8 MPa(5 and 8 bar). The limit value of temperature is generally between 400°C. and 430° C., the temperature corresponding, here and hereinafter, tothe average measured temperature of the catalyst bed. Another limitvalue for the temperatures, indicating that deactivation is reached(lower level of exothermic reactions), is that the temperaturedifference (ΔT) on a catalyst bed becomes less than 5° C., regardless ofthe average temperature value.

FIG. 2 shows the hydrotreating process according to the presentinvention using a system of two switchable reactors each containing twocatalyst beds and in which the catalyst beds can be by-passed. In thecase shown in FIG. 2 the process comprises a series of cycles eachhaving six successive steps, steps a), b), c) and d) being identical tothe process described in FR2681871:

-   -   a step a) during which the feed passes successively through all        the catalyst beds of reactor R1 a, then all the catalyst beds of        reactor R1 b,    -   a step a′) (by-pass step) during which the feed by-passes the        deactivated and/or clogged catalyst bed A1 of the first reactor        R1 a and is introduced into the next catalyst bed A2 downstream,        then passes through all the catalyst beds of reactor R1 b,    -   a step b), after deactivation and/or clogging of bed A2, during        which the feed passes through all the catalyst beds of reactor        R1 b only, reactor R1 a being by-passed for catalyst        regeneration and/or replacement,    -   a step c) during which the feed passes successively through all        the catalyst beds of reactor R1 b, then all the catalyst beds of        reactor R1 a,    -   a step c′) (by-pass step) during which the feed by-passes the        deactivated and/or clogged catalyst bed B1 of reactor R1 b and        is introduced into the next catalyst bed B2 downstream, then        passes through all the catalyst beds of reactor R1 a,    -   a step d), after deactivation and/or clogging of bed B2, during        which the feed passes through all the catalyst beds of reactor        R1 a only, reactor R1 b being by-passed for catalyst        regeneration and/or replacement.

Thus, at step a) the feed is introduced via line 3 and lines 21 and 21′,having an open valve V1, into the guard reactor R1 a and passes throughthe fixed beds A1 and A2. During this period, valves V1′, V3, V3′, V4and V5 are closed. The effluent from reactor R1 a is sent via pipe 23,pipe 26, having an open valve V2, and pipe 22′ to the guard reactor R1 band passes through the catalyst beds B1 and B2. The effluent is removedfrom reactor R1 b via pipes 24 and 24′, having an open valve V6, andpipe 13.

Gradually, the catalyst beds, and in particular the first catalyst bed,on being brought into contact with the feed (A1 of reactor R1 a), willbecome clogged and/or deactivated. The moment when it is considered thatthe first catalyst bed brought into contact with the feed is deactivatedand/or clogged is measured from the head loss Δp and/or temperature of aguard zone. A maximum tolerable value for the head loss and/ortemperature from which it is necessary either to by-pass the deactivatedand/or clogged catalyst bed, or to proceed with replacement of thecatalyst in the reactor, is defined beforehand. Each time that thislimit value is reached, the catalyst bed that is clogged and/ordeactivated is by-passed by introducing the feed by a by-pass deviceoutside the reactor onto the next catalyst bed not yet deactivatedand/or clogged downstream of said reactor.

Thus, according to FIG. 2, once the maximum value of head loss and/or oftemperature is reached, valve V1 is closed and the feed is introducedvia line 31, having an open valve V1′, onto the next catalyst bed A2 inreactor R1 a (step a′). The deactivated and/or clogged catalyst bed A1is therefore by-passed. Catalyst bed A2 is much less clogged and/ordeactivated than the first bed A1, permitting a considerable increase inthe length of the first period, by using the lower bed A2 for a longertime.

Gradually, this next catalyst bed A2 is also clogged and/or deactivated.When the maximum value of head loss and/or of temperature is reached,step b) is then carried out, during which the feed passes through allthe catalyst beds of reactor R1 b only, reactor R1 a being by-passed forcatalyst regeneration and/or replacement. Thus, during step b), valvesV1, V1′, V2, V3′, V4 and V5 are closed and the feed is introduced vialine 3 and lines 22 and 22′, having an open valve V3, into reactor R1 b.During this period the effluent from reactor R1 b is removed via pipes24 and 24′, having an open valve V6, and via pipe 13.

After reconnection of reactor R1 a, of which the catalyst wasregenerated or replaced downstream of reactor R1 b, step c) of theprocess is then carried out, during which the feed passes successivelythrough reactor R1 b, then reactor R1 a. Thus, during step c), valvesV1, V1′, V2, V3′ and V6 are closed and valves V3, V4 and V5 are open.The feed is introduced via line 1 and lines 22 and 22′ into reactor R1b. The effluent from reactor R1 b is sent via pipe 24, pipe 27, havingan open valve V4, and pipe 21′ to the guard reactor R1 a. The effluentfrom reactor R1 a is removed via pipes 23 and 23′, having open valve V5,and via pipe 13.

Gradually, the catalyst beds, and in particular the first bed B1 ofreactor R1 b, will become clogged and/or deactivated. Then, just as instep a′), by-passing of the deactivated and/or clogged catalyst bed B1,called step c′), is carried out. Thus, according to FIG. 2, once themaximum value of head loss and/or of temperature is reached, valve V3 isclosed and the feed is introduced into the reactor via line 32, havingan open valve V3′, onto the next bed B2 in reactor R1 b. The deactivatedand/or clogged catalyst bed B1 is therefore by-passed. The catalyst bedB2 is much less clogged and/or deactivated than the first catalyst bedB1, permitting a considerable increase in the length of the thirdperiod, by using the lower bed B2 for a longer time.

Gradually, this next catalyst bed B2 is also clogged and/or deactivated.When the maximum value of head loss and/or of temperature is reached,step d) is then carried out, during which the feed passes through allthe catalyst beds of reactor R1 a only, reactor R1 b being by-passed forcatalyst regeneration and/or replacement. During this step valves V1′,V2, V3, V3′, V4 and V6 are closed and valves V1 and V5 are open. Thefeed is introduced via line 3 and lines 21 and 21′ into reactor R1 a.During this period the effluent from reactor R1 a is removed via pipes23 and 23′, having open valve V5, and via pipe 13.

After catalyst regeneration and/or replacement in reactor R1 b, thisreactor is reconnected downstream of reactor R1 a and the cycle beginsagain.

The operations on the valves of the unit permitting functioning of thetwo switchable reactors having two catalyst beds that can be by-passedaccording to the present invention are presented in Table 2.

TABLE 2 Operations on the valves for the system of switchable reactorswith external by-pass (according to the invention) Step in CycleIntervention V1 V1′ V2 V3 V3′ V4 V5 V6 a R1A + R1B — O* C** O C C C C Oa′ R1A + R1B — C O O C C C C O b R1B R1A C C C O C C C O c R1B + R1A — CC C O C O O C c′ R1B + R1A — C C C C O O O C d R1A R1B O C C C C C O C aR1A + R1B — O C O C C C C O *O = open, **C = closed

The system of switchable reactors with external by-pass can be extendedto reactors having more than two catalyst beds, for example 3, 4 or 5catalyst beds. In this case, the external by-pass feeds, by additionallines and valves, respectively, the next catalyst bed downstream of thedeactivated and/or clogged catalyst bed once the maximum value of headloss and/or of temperature is reached. Thus, step a′) or c′) as definedabove is repeated. This by-passing of catalyst beds can continue untilthe last catalyst bed of the reactor in the direction of flow isdeactivated and/or clogged. It is then necessary to replace the catalystcontained in the reactor. FIG. 3 shows the hydrotreating processaccording to the present invention using a system of two switchablereactors each containing three catalyst beds A1, A2, A3 and B1, B2 andB3 respectively. In FIG. 3, steps a), a′), b), c) c′) and d) (andreference symbols) are identical to FIG. 2, except that steps a′) andc′) are repeated. This repetition only will be described for thisfigure.

Thus, during step a′), once catalyst bed A1, and then catalyst bed A2are deactivated and/or clogged, valve V1′ is closed and the feed isintroduced via line 33, having an open valve V1″, onto the next catalystbed A3 in reactor R1 a. When this third bed A3 is also clogged and/ordeactivated, step b) (replacement/regeneration of reactor R1 a) is thencarried out. Similarly, during step c′), once catalyst bed B1, and thencatalyst bed B2 are deactivated and/or clogged, valve V3′ is closed andthe feed is introduced via line 34, having an open valve V3″, onto thenext catalyst bed B3 in reactor R1 b. When this third bed B3 is alsoclogged and/or deactivated, step d) (replacement/regeneration of reactorR1 b) is then carried out.

In a preferred embodiment, the catalyst beds contained in a guard zonecan be of different or identical volumes but with the condition that thevolume of the last bed is greater than each volume of the other beds.Preferably, the catalyst beds in one and the same guard zone havevolumes that increase in the direction of flow. In fact, since cloggingand/or deactivation occurs mainly on the first catalyst bed, it isadvantageous to minimize the volume of this first bed.

The volume of each bed can be defined as follows:

Each guard zone has n beds, each bed i having a volume V_(i), the totalcatalyst volume of the reactor V_(tot) being the sum of the volumesV_(i) of the n beds:

Vtot=V ₁ + . . . V _(i) +V _(i+1) . . . +V _(n−1) +V _(n)

Each volume V_(i) of a bed i included in the n−1 first beds of the guardzone is defined between 5% of the total volume V_(tot) and thepercentage resulting from the total volume V_(tot) divided by the numberof beds n:

5% Vtot≧Vi≧(Vtot/n)

For two consecutive beds i and i+1, the volume of the first bed V_(i) isless than or equal to the volume of the next bed V_(i+1), except for thelast two consecutive beds V_(n−1) and where the volume of thepenultimate bed V_(n−1) is strictly less than the volume of the last bedV_(n).

In the case of two catalyst beds in a guard zone, the volume V1 of thefirst bed is thus between 5 and 49%, the volume of the second bed isbetween 51 and 95%.

In the case of three catalyst beds in a guard zone, the volume V1 of thefirst bed is thus between 5 and 33%, the volume V2 of the second bed isbetween 5 and 33% and the volume V3 of the third bed is between 34 and90%.

The maximum volume of the by-passed catalyst bed(s) in a guard zoneduring steps a′) and c′), also called “by-passable fraction”, is the sumof the volumes V₁+ . . . V_(i)+V_(i+1) . . . +V_(n−1) of the n−1 beds(or the total volume minus the volume of the last bed n). This maximumvolume of the by-passed catalyst bed(s) is defined as being less thanthe percentage expressed by the formula ((n−1) V_(tot))/n, n being thebed number in a guard zone, V_(tot) being the total catalyst volume ofthe guard zone.

Starting from a certain value of by-passed fraction, generally greaterthan or equal to ((n−1) V_(tot))/n, the quantity of fouling material andmetals accumulated in the last bed of the first reactor and thataccumulated in the second reactor become very similar. Thus, a head lossand/or temperature increase may be observed, reaching the maximum valuein the two reactors almost at the same time, and can lead to continuousmalfunction of the reactors. It is thus important to have a minimumvolume that cannot be by-passed in the first reactor to protect thesecond reactor and have time to regenerate the first reactor beforethere is an increase in head loss and/or temperature in the secondreactor. In order to maximize the duration of a step during which thefeed passes successively through all the reactors, it is thereforebeneficial to by-pass a substantial quantity of the reactor, but withoutexceeding a limit value.

In a preferred embodiment, a catalyst conditioning section is used,allowing these guard zones to be switched while in operation, i.e.without stopping the operation of the unit: first, a system thatoperates at moderate pressure (from 10 to 50 bar, but preferably from 15to 25 bar) allows the following operations to be performed on thedisconnected guard reactor: washing, stripping, cooling, beforedischarging the used catalyst; then heating and sulphurization afterloading the fresh catalyst; then another system forpressurization/depressurization, with gate valves of appropriate design,permits efficient switching of these guard zones without stopping theunit, i.e. without affecting its operating factor, since all theoperations of washing, stripping, discharge of the used catalyst,loading of the fresh catalyst, heating, and sulphurization are carriedout on the disconnected reactor or guard zone. Alternatively, apre-activity catalyst can be used in the conditioning section so as tosimplify the procedure for switching while in operation.

Each guard zone contains at least two catalyst beds (for example 2, 3,4, or 5 catalyst beds). Each catalyst bed contains at least one catalystlayer containing one or more catalysts, optionally supplemented with atleast one inert layer. The catalysts used in the catalyst bed(s) may beidentical or different.

The hydrotreating process using switchable reactors with externalby-pass can thus greatly increase the duration of a cycle. During theby-pass steps the feed has a shorter residence time in the switchablereactors because of the by-pass. In order to maintain a constant degreeof hydrotreating at the outlet of the last reactor, the temperature inthe guard zones is thus gradually increased. The latter is alsoincreased overall during the cycle to counteract the catalystdeactivation. However, this temperature increase promotes the depositionof coke, accelerating the processes of clogging. Thus, to limit anexcessive temperature rise, the by-passed fraction must be all the morerestricted. The reactor fraction that is by-passed is thus based onoptimization between the gain in cycle time and limited temperaturerise.

According to a preferred variant, at the entrance of each guard zone thefeed passes through a filtering distributor plate composed of a singlestage or of two successive stages, said plate is situated upstream ofthe catalyst beds, preferably upstream of each catalyst bed. Thisfiltering distributor plate, described in patent US2009177023, makes itpossible to trap the clogging particles contained in the feed by meansof a special distributor plate comprising a filtering medium. Thus, thefiltering plate makes it possible to increase the gain of cycle time inthe hydrotreating process using switchable guard zones. This filteringplate simultaneously provides distribution of the gas phase (hydrogenand the gaseous portion of the feed) and the liquid phase (the liquidportion of the feed) feeding the reactor while providing a filtrationfunction with respect to the impurities contained in the feed. Moreover,the filtering plate ensures a more uniform distribution of the mixtureover the whole surface of the catalyst bed and limits the problems ofpoor distribution during the phase of clogging of the plate itself.

More precisely, the filtering plate is a device for filtration anddistribution, said device comprising a plate situated upstream of thecatalyst bed, said plate consisting of a base that is approximatelyhorizontal and integral with the walls of the reactor and to whichapproximately vertical chimneys are fixed, open at the top for admissionof the gas, and at the bottom for removing the gas-liquid mixtureintended to feed the catalyst bed situated downstream, said chimneysbeing pierced over a certain fraction of their height by a continuouslateral slit or by lateral orifices for admission of liquid, said platesupporting a filtering bed surrounding the chimneys, and said filteringbed consisting of at least one layer of particles of size less than orequal to the size of the particles of the catalyst bed. The filteringbed consists of particles that are generally inert but can also compriseat least one layer of catalyst identical to or belonging to the samefamily as the catalyst of the catalyst bed. This last-mentioned variantmakes it possible to reduce the volume of catalyst beds in the reactor.

The filtering distributor plate can also comprise two stages and becomposed of two successive plates: the first plate supporting a guardbed composed of internal particles and of at least one layer of catalystidentical to or belonging to the same family as the catalyst of thecatalyst bed. This plate is described in patent US2009177023. The bed isarranged on a grating, the liquid phase flows through the guard bed andthe gas through the chimneys passing through the guard bed and the firstplate. At the end of clogging the liquid and the gas flow simultaneouslythrough the chimneys while allowing the second plate to continue toprovide its distribution function. The second plate provides thefunction of distribution of the gas and liquid: it can be composed ofchimneys with lateral perforations for passage of the liquid or becomposed of bubble-caps or vapour-lift.

According to another variant, the hydrotreating process according to thepresent invention can comprise more than two switchable reactors (forexample 3, 4 or 5) functioning according to the same principle ofswitching and by-pass, each switchable reactor having at least twocatalyst beds.

FIG. 4 shows the case of three guard zones each having two catalystbeds. The process will comprise, in its preferred embodiment, a seriesof cycles each having nine successive steps:

-   -   a step a) during which the feed passes successively through all        the catalyst beds of reactor R1 a, then all the catalyst beds of        reactor R1 b and finally all the catalyst beds of reactor R1 c,    -   a step a′) (by-pass step) during which the feed by-passes the        deactivated and/or clogged catalyst bed A1 of the first reactor        R1 a and is introduced into the next catalyst bed A2 downstream        of reactor R1 a, then passes through all the catalyst beds of        reactor R1 b and finally all the catalyst beds of reactor R1 c,    -   a step b) during which the feed passes through all the catalyst        beds of reactor R1 b, then all the catalyst beds of reactor R1        c, reactor R1 a being by-passed for catalyst regeneration and/or        replacement,    -   a step c) during which the feed passes successively through all        the catalyst beds of reactor R1 b then all the catalyst beds of        reactor R1 c and finally all the catalyst beds of reactor R1 a,    -   a step c′) (by-pass step) during which the feed by-passes the        deactivated and/or clogged catalyst bed B1 of the second reactor        R1 b and is introduced into the next catalyst bed B2 downstream        of reactor R1 b, then passes through all the catalyst beds of        reactor R1 c, and finally all the catalyst beds of reactor R1 a,    -   a step d) during which the feed passes through all the catalyst        beds of reactor R1 c, then all the catalyst beds of reactor R1        a, reactor R1 b being by-passed for catalyst regeneration and/or        replacement,    -   a step e) during which the feed passes successively through all        the catalyst beds of reactor R1 c then all the catalyst beds of        reactor R1 a and finally all the catalyst beds of reactor R1 b,    -   a step e′) (by-pass step) during which the feed by-passes the        deactivated and/or clogged catalyst bed C1 of third reactor R1 c        and is introduced into the next catalyst bed C2 downstream of        reactor R1 c, then passes through all the catalyst beds of        reactor R1 a, and finally all the catalyst beds of reactor R1 b,    -   a step f) during which the feed passes through all the catalyst        beds of reactor R1 a, then all the catalyst beds of reactor R1        b, reactor R1 c being by-passed for catalyst regeneration and/or        replacement.

In the case shown schematically in FIG. 4 the process functions in anequivalent manner to that described in connection with FIG. 2 (thereference symbols for the lines have been omitted for reasons oflegibility).

During step a), valves V1, V2, V7 and V8 are open and valves V1′, V3,V3′, V5, V6, V9, V10 and V10′ are closed.

During step a′), valves V1′, V2, V7, V8 are open and valves V1, V3, V3′,V5, V6, V9, V10 and V10′ are closed.

During step b), valves V3, V7 and V8 are open and valves V1, V1′, V2,V3′, V5, V6, V9, V10 and V10′ are closed.

During step c), valves V3, V7, V9 and V5 are open and valves V1, V1′,V2, V3′, V6, V8, V10 and V10′ are closed.

During step c′), valves V3′, V7, V9 and V5 are open and valves V1, V1′,V2, V3, V6, V8, V10 and V10′ are closed.

During step d), valves V10, V9 and V5 are open and valves V1, V1′, V2,V3, V3′, V6, V7, V8 and V10′ are closed.

During step e), valves V10, V9, V2 and V6 are open and valves V1, V1′,V3, V3′, V5, V7, V8 and V10′ are closed.

During step e′), valves V10′, V9, V2 and V6 are open and valves V1, V1′,V3, V3′, V5, V7, V8 and V10 are closed.

During step f), valves V1, V2 and V6 are open and valves V1′, V3, V3′,V5, V7, V8, V9, V10 and V10′ are closed.

The different variants of the process described above for a system oftwo switchable reactors having two catalyst beds also apply to a systemhaving more than two switchable reactors. These different variants arein particular: the conditioning system, the possibility of having morethan two catalyst beds per reactor, the possibility of having beds withdifferent volumes as defined above, the volume of the by-passed catalystbed(s) in one guard zone being less than ((n−1)Vtot)/n, maintaining thedegree of hydrotreating by raising the temperature, integration of afiltering plate at the entrance of each reactor upstream of the firstcatalyst bed, preferably upstream of each catalyst bed.

The process according to the invention can advantageously be carried outat a temperature between 320° C. and 430° C., preferably 350° C. to 410°C., at a hydrogen partial pressure advantageously between 3 MPa and 30MPa, preferably between 10 and 20 MPa, at a space velocity (HSV)advantageously between 0.05 and 5 volumes of feed per volume of catalystand per hour, and with a ratio of hydrogen gas to liquid hydrocarbonfeed advantageously between 200 and 5000 normal cubic metres per cubicmetre, preferably 500 to 1500 normal cubic metres per cubic metre. Thevalue of HSV of each switchable reactor in operation is preferably fromabout 0.5 to 4 h⁻¹ and most often from about 1 to 2 h⁻¹. The overallvalue of HSV of the switchable reactors and that of each reactor isselected so as to achieve maximum HDM while controlling the reactiontemperature (limiting the exothermic effect).

The hydrotreating catalysts used are preferably known catalysts and aregenerally granular catalysts comprising, on a support, at least onemetal or metal compound having a hydro-dehydrogenating function. Thesecatalysts are advantageously catalysts comprising at least one groupVIII metal, generally selected from the group comprising nickel and/orcobalt, and/or at least one group VIB metal, preferably molybdenumand/or tungsten. The support used is generally selected from the groupcomprising alumina, silica, silica-aluminas, magnesia, clays andmixtures of at least two of these minerals.

Prior to injection of the feed, the catalysts used in the processaccording to the present invention are preferably subjected to asulphurization treatment for transforming, at least partly, the metallicspecies to sulphide before they are brought into contact with the feedto be treated. This treatment of activation by sulphurization is wellknown to a person skilled in the art and can be carried out by anymethod already described in the literature, either in situ, i.e. in thereactor, or ex situ.

The feeds treated in the process according to the invention areadvantageously selected from atmospheric residues, vacuum residues fromdirect distillation, crude oils, topped crude oils, deasphalted oils,residues from conversion processes such as for example those originatingfrom coking, from fixed-bed, ebullating-bed, or moving-bedhydroconversion, heavy oils of any origin and in particular thoseobtained from oil sands or oil shale, used alone or mixed. These feedscan advantageously be used as they are or diluted with a hydrocarbonfraction or a mixture of hydrocarbon fractions that can be selected fromthe products obtained from a fluid catalytic cracking (FCC) process, alight cut of oil (Light Cycle Oil, LCO), a heavy cut of oil (Heavy CycleOil, HCO), a decanted oil (DO), a residue from FCC, or that can beobtained from distillation, the gas oil fractions, in particular thoseobtained by vacuum distillation (Vacuum Gas Oil, VGO). The heavy feedscan also advantageously comprise cuts obtained from the coalliquefaction process, aromatic extracts, or any other hydrocarbon cutsor also non-petroleum feeds such as gaseous and/or liquid derivatives(containing little if any solids) from thermal conversion (with orwithout catalyst and with or without hydrogen) of coal, biomass orindustrial waste, such as for example recycled polymers.

Said heavy feeds generally have more than 1 wt. % of molecules having aboiling point above 500° C., a content of metals Ni+V above 1 ppm byweight, preferably above 20 ppm by weight, a content of asphaltenes,precipitated in heptane, above 0.05 wt. %, preferably, above 1 wt. %.

The hydrotreating process according to the invention makes it possibleto effect 50% or more of HDM of the feed at the outlet of the switchablereactors (and more precisely from 50 to 95% of HDM) owing to the HSVselected and the efficiency of the HDM catalyst.

The hydrotreating process according to the invention using the system ofswitchable guard zones including at least one by-pass stepadvantageously precedes a fixed bed or ebullating bed process forhydrotreating heavy hydrocarbon feeds.

Preferably, it precedes the applicant's Hyvahl-F™ process comprising atleast one hydrodemetallization step and at least onehydrodesulphurization step. The process according to the invention ispreferably integrated upstream of the HDM section, the switchablereactors being used as guard beds. In the case shown in FIG. 1, the feed1 enters the switchable guard reactor(s) via pipe 1 and leaves saidreactor(s) via pipe 13. The feed leaving the guard reactor(s) enters,via pipe 13, the hydrotreating section 14 and more precisely the HDMsection 15 comprising one or more reactors. The effluent from the HDMsection 15 is withdrawn via pipe 16, and then sent to the HDT section 17comprising one or more reactors. The final effluent is withdrawn viapipe 18.

The present invention also relates to an installation (FIG. 2) forimplementing the process according to the invention comprising at leasttwo fixed bed reactors (R1 a, R1 b) arranged in series and eachcontaining at least two catalyst beds (A1,A2; B1,B2), the first bed ofeach reactor having at least one inlet pipe for a gas (not shown) and aninlet pipe for a hydrocarbon feed (21, 22), said inlet pipes for thefeed each containing a valve (V1, V3) and being connected by a commonpipe (3), each reactor having at least one outlet pipe (23, 24) eachcontaining a valve (V5, V6) for removal of the effluent, the outlet pipeof each reactor (23, 24) being connected by an additional pipe (26, 27)having a valve (V2, V4) to the inlet pipe (22, 21) of the feed of thereactor downstream, characterized in that the installation furthercomprises, for each reactor, a feed inlet pipe for each catalyst bed(31, 32), said pipes each having a valve (V1′, V3′) and being connectedto said inlet pipe for the hydrocarbon feed of the first bed (21, 22),and each valve of the installation being able to be opened or closedseparately.

According to a preferred variant, the installation comprises a filteringdistributor plate composed of a single stage or of two successive stagesat the entrance of each reactor, situated upstream of the catalyst beds,preferably upstream of each catalyst bed.

Example 1 Not According to the Invention

The feed consists of a mixture (70/30 wt. %) of atmospheric residue (AR)of Middle East origin (Arabian Medium) and of a vacuum residue (VR) ofMiddle East origin (Arabian Light). This mixture is characterized by ahigh viscosity (0.91 cP) at ambient temperature, a density of 994 kg/m³,high contents of Conradson carbon (14 wt. %) and asphaltenes (6 wt. %)and a high level of nickel (22 ppm by weight), vanadium (99 ppm byweight) and sulphur (4.3 wt. %).

The hydrotreating process is carried out according to the processdescribed in FR2681871 and comprises the use of two switchable reactors.The two reactors are loaded with a CoMoNi/alumina hydrodemetallizationcatalyst. A cycle is defined as integrating the steps from a) to d). Thedeactivation time and/or clogging time is reached when the head lossreaches 0.7 MPa (7 bar) and/or the average temperature of a bed reaches405° C. and/or when the temperature difference on a catalyst bed becomesless than 5° C.

The process is carried out at a pressure of 19 MPa, a temperature atreactor inlet at the start of the cycle of 360° C. and at the end of thecycle of 400° C., and an HSV=2h⁻¹ per reactor, allowing a degree ofdemetallization close to 60% to be maintained.

Table 3 and FIG. 5 show the operating time (in days) for the processaccording to FR 2681871 (without by-pass). Thus, according to FIG. 5,the curve of reactor R1 a according to the state of the art (base caseR1 a) shows, at the start of the cycle, an increase of head loss in thefirst reactor R1 a up to its maximum tolerable value (Δp=0.7 MPa or 7bar), after which catalyst replacement is required. In the case of thestate of the art (FR268187), the operating time of reactor R1 a istherefore 210 days. At the time of replacement of the catalyst ofreactor R1 a, the head loss in reactor R1 b reached about 3 bar. Duringthe next phase in which the feed passes through reactor R1 b and thenreactor R1 a containing fresh catalyst, the head loss of reactor R1 bincreases up to the maximum tolerable value, which is reached after 320days of operation. A second cycle can be envisaged on these switchablereactors, replacing the catalyst of reactor R1 b.

The deactivation time and/or clogging time (or the operating time) ofthe first zone is therefore 210 days. Overall, a cycle time of 320 daysfor the first cycle and of 627 days for two cycles is observed.

Example 2 According to the Invention

The hydrotreating process is repeated with the same feed and under thesame operating conditions and with the same catalyst according toexample 1, except that the process comprises the use of two switchablereactors, each reactor containing two catalyst beds, the first catalystbed representing a volume of 20%, and the second representing a volumeof 80% (by-pass of 20%), and the process according to the invention iscarried out. A cycle is defined as integrating the steps from a) to d).The deactivation time and/or clogging time is reached when the head lossreaches 0.7 MPa (7 bar) and/or the average temperature of a bed reaches405° C. and/or when the temperature difference on a catalyst bed becomesless than 5° C. The degree of HDM is maintained at 60%.

Table 3 and FIG. 5 show the gain in operating time (in days) for theprocess according to the invention with a by-passed fraction of 20% ineach reactor.

TABLE 3 Gain in operating time (days) without external by-pass(according to FR2681871) and with a by-pass of 20% in each reactor. Base(by-pass 0%) (not By-Pass 20% according to the (according to the Caseinvention) invention) Duration R1-A Cycle 1 210 d 252 d Duration R1-BCycle 1 320 d 380 d Total gain End 1 cycle —  60 d Duration R1-A Cycle 2487 d 577 d Duration R1-B Cycle 2 627 d 741 d Total gain End 2 cycles —114 d

It can therefore be seen that the hydrotreating process integrating aby-passed fraction of 20% makes it possible to increase the duration ofa first cycle by 60 days (i.e. by 18.75%) and by 114 days for two cycles(i.e. by 18.2%) while maintaining a degree of HDM of 75%, equivalent tothe degree of HDM according to the process without external by-pass.

FIG. 5 shows the variation of head loss during the time measured inreactors R1 a and R1 b without external by-pass (according to FR2681871,curves for the Base Cases R1 a and R1 b) and in reactors R1 a and R1 bwith an external by-pass of 20% (according to the invention, curves PRSByP R1 a and R1 b).

Thus, according to FIG. 5, the curve of reactor R1 a (curve PRS ByP R1a) shows, at the start of the cycle, an increase of head loss in thefirst reactor R1 a up to its maximum tolerable value (Δp=0.07 MPa or 7bar). When this value is reached, the first bed is by-passed and thefeed is introduced onto the second bed A2 of reactor R1 a. The head lossin the reactor then drops suddenly (hook in curve PRS ByP R1 a), withoutreturning to the initial head loss, to gradually increase again up tothe point where the next (second) bed is clogged and the limit value ofthe head loss is reached again. The gain in time obtained at the end ofstep a′) is then Δt_(C1-R1a) (32 days). The head loss of reactor R1 athen drops abruptly because the system passes to step b), during whichthe catalyst of reactor R1 a is replaced. The feed then only passesthrough reactor R1 b, and then R1 b and R1 a after replacement.

Curve R1 b (curve PRS ByP R1 b) shows the head loss of the secondreactor R1 b as a function of time. The same phenomenon of gain of timeby external by-pass is observed at the end of step c′): Δt_(C2-R1b) (60days).

FIG. 2 also shows a second cycle of switchable reactors. The gain oftime after 2 successive cycles is then Δt_(C2-R1b) (114 days). It can beseen that the more cycles there are, the larger the gain of time.

1. Process for hydrotreating a heavy hydrocarbon fraction containingasphaltenes, sediments, sulphur-containing, nitrogen-containing andmetallic impurities, in which the feed of hydrocarbons and hydrogen ispassed, under conditions of hydrotreating, over a hydrotreatingcatalyst, in at least two fixed bed hydrotreating guard zones eachcontaining at least two catalyst beds, the guard zones being arranged inseries to be used cyclically, consisting of successive repetition ofsteps b), c) and c′) defined below: a step a) during which the feedpasses through all the catalyst beds of the guard zones for a period atmost equal to the deactivation time and/or clogging time of a guardzone, a step a′) during which the feed is introduced, by-passing thedeactivated and/or clogged catalyst bed, onto the next catalyst bed notyet deactivated and/or clogged of the same guard zone for a period atmost equal to the deactivation time and/or clogging time of a guardzone, step a′) being repeated until the feed is introduced onto the lastcatalyst bed not yet deactivated and/or clogged of the same guard zonefor a period at most equal to the deactivation time and/or cloggingtime, a step b) during which the deactivated and/or clogged guard zoneis by-passed and the catalyst that it contains is regenerated and/orreplaced with fresh catalyst and during which the other guard zone(s)are used, a step c) during which the feed passes through all thecatalyst beds of the guard zones, the guard zone of which the catalystwas regenerated during the preceding step being reconnected so as to bedownstream of all the other guard zones and said step being continuedfor a period at most equal to the deactivation time and/or clogging timeof a guard zone, a step c′) during which the feed is introduced onto thenext catalyst bed not yet deactivated and/or clogged of the same guardzone for a period at most equal to the deactivation time and/or cloggingtime of a guard zone, step c′) being repeated until the feed isintroduced onto the last catalyst bed not yet deactivated and/or cloggedof the same guard zone for a period at most equal to the deactivationtime and/or clogging time of a guard zone.
 2. Process according to claim1 in which each guard zone has n beds, each bed i having a volume V_(i),the total catalyst volume of the guard zone V_(tot) being the sum of thevolumes V_(i) of the n beds; each volume V_(i) of a bed i included inthe n−1 first beds of the guard zone has a volume V_(i) defined between5% of the total volume V_(tot) and the percentage resulting from thetotal volume V_(tot) divided by the number of beds n; and in which fortwo consecutive beds i and i+1, the volume of the first bed V_(i) isless than or equal to the volume of the next bed V_(i+1), except for thelast two consecutive beds V_(n−1) and V_(n) where the volume of thepenultimate bed V_(n−1) is strictly less than the volume of the last bedV_(n).
 3. Process according to claim 1 in which during steps a′) and c′)the maximum volume of the by-passed catalyst bed(s) in a guard zone isdefined as less than the volume given by the formula ((n−1) V_(tot))/n,n being the total number of catalyst beds, V_(tot) being the totalcatalyst volume of the guard zone which is defined by the sum of thevolumes of the n catalyst beds of the guard zone.
 4. Process accordingto claim 1 in which the degree of hydrotreating is maintained by atemperature increase.
 5. Process according to claim 1 in which, at theentrance of each guard zone, the feed passes through a filteringdistributor plate composed of a single stage or of two successivestages, said plate is situated upstream of the catalyst beds.
 6. Processaccording to claim 1 in which the feed passes through a filteringdistributor plate upstream of each catalyst bed of a guard zone. 7.Process according to claim 1, characterized in that it precedes a fixedbed or ebullating bed hydrotreating process.
 8. Installation forimplementing the process according to claim 1 comprising at least twofixed bed reactors (R1 a, R1 b) arranged in series and each containingat least two catalyst beds (A1, A2; B1, B2), the first bed of eachreactor having at least one inlet pipe for a gas and an inlet pipe for ahydrocarbon feed (21, 22), said feed inlet pipes each containing a valve(V1, V3) and being connected by a common pipe (3), each reactor havingat least one outlet pipe (23, 24) each containing a valve (V5, V6) forremoval of the effluent, the outlet pipe of each reactor (23, 24) beingconnected by an additional pipe (26, 27) having a valve (V2, V4) to thefeed inlet pipe (22, 21) of the reactor downstream, characterized inthat the installation further comprises, for each reactor, a feed inletpipe for each catalyst bed (31, 32), said pipes each having a valve(V1′, V3′) and being connected to said inlet pipe for the hydrocarbonfeed of the first bed (21, 22), each valve of the installation beingable to be opened or closed separately.
 9. Installation according toclaim 8, characterized in that it comprises a filtering distributorplate composed of a single stage or of two successive stages at theentrance of each reactor, said plate is situated upstream of thecatalyst beds.
 10. Installation according to claim 8, characterized inthat it comprises a filtering distributor plate composed of a singlestage or of two successive stages upstream of each catalyst bed.